Adjust gdb.python/py-events.exp for Cygwin/MinGW, "info proc" => "inferior"

The testcase is using "info proc" to extract the inferior's process
ID.  But "info proc" does not exist on all targets, including Windows.
Switch to using the get_inferior_pid routine from lib/gdb.exp, which
uses "inferior" instead.

With this fixed, the testcase passes cleanly on Cygwin.  I haven't
tested on MinGW (I'm not set up for Python testing there currently),
but at least (since the previous patches) the test should be able to
compile & run there now.

Also tested on x86_64-unknown-linux-gnu.

Change-Id: If6ff482ceb011d9afe5ed40ef7e4e2f2cad8cae8